In current mobile communication techniques, the microwave filter device has already become an indispensable and important component. With the advantage of excellent electromagnetic shielding, compact structure, low pass-band signal loss, small size and high power capacity, the metallic cavity filter has been selected as the first choice of the mobile communication base station transmitting filter for a long time.
For a combiner featuring several pass-bands, the combiner is mostly constructed of a dually layered cavity. The combiner with a dually layered cavity includes a common port in the form of a joint by which an upper filter path and a lower filter path are connected with each other. In a traditional design, a joint is used to weld two wires on it; one of the wires is connected to a first harmonic cavity of the upper filter path, while the other wire is connected to a first harmonic cavity of the lower filter path so that the upper filter path and the lower filter path are coupled together. Alternatively, a common harmonic cavity may be disposed in a middle location between the upper filter path and the lower filter path to realize coupling of the upper filter path and the lower filter path together through a common cavity.
In the first coupling manner it is necessitated that two wires be welded together (one wire is welded to the upper harmonic cavity, while the other wire is welded to the lower harmonic cavity), resulting in time and labor consumption. In addition, nonlinear factors of the cavity are also increased inevitably due to increase of welding spots.
Using the common harmonic cavity results in addition of another harmonic cavity of the combiner and accordingly, it also results in an increase in insertion loss. The loss outweighs the gain. Moreover, a common cavity, which is located in a middle position between the upper cavity and the lower cavity, is utilized, leading to difficulty in manufacture and complicated coupling between the ports and difficult in tuning.